Fireworks CSS and Images export does not support Javascript. Same with States, if I recall. CSS and Images will not export out anything but the current state. You would have to add that code (CSS or JS) in by using Dreamweaver or another editor.

If properly set up, you may be able to export the entire page as CSS and images, then only have to add in the CSS to create the mouseover effect, after they've been exported as graphics.